Cement is used in the casing and liners of a wellbore. The annular space between the casing/lining and the wellbore is filled with a predetermined quantity of a cement mixture, which after hardening retains the casing/liner in place in the wellbore. The cement mixture is pumped in at the top end of the casing or liner, down to the lower end thereof and out into and up the annular space on the outside of the casing/liner.
Cementing is employed during many phases of wellbore operations. For example, cement may be employed to cement or secure various casing strings and/or liners in a well. Cementing may also be used to repair casing and/or to achieve formation isolation. Additionally, cementing may be employed during well abandonment. Cement operations performed in wellbores under these high stress conditions present problems including difficulty in obtaining wellbore isolation and maintaining the mechanical integrity of the wellbore.
In essence, cement is placed in the annulus created between the outside surface of a pipe string and the inside formation surface or wall of a wellbore in order to form a sheath to seal off fluid and/or solid production from formations penetrated by the wellbore. Cementing allows a wellbore to be selectively completed to allow production from, or injection into, one or more productive formations penetrated by the wellbore. Cement may be used for purposes including sealing off perforations, repairing casing leaks, plugging back or sealing off the lower section of a wellbore, or sealing the interior of a wellbore during abandonment operations.
Once established, this isolation may be impacted by the particular stresses associated with the environment found in the wellbore during operations. The cement sheath may be exposed to stresses imposed by well operations such as perforating, hydraulic fracturing, or high temperature-pressure differentials.
Furthermore, well cement compositions may be brittle when cured. These cement compositions may fail due to tensional and compressional stresses that are exerted on the set cement. These wellbore cements may be subjected to axial, shear, and compressional stresses. Relatively high temperatures may induce stress conditions and/or relatively high fluid pressures encountered inside cemented wellbore pipe strings during operations such as perforating, stimulation, injection, testing, or production. Moreover, stress conditions may be induced or aggravated by fluctuations or cycling in temperature or fluid pressures during similar operations. In addition, variations in temperature and internal pressure of the wellbore pipe string may result in radial and longitudinal pipe expansion and/or contraction which tends to place stress on the annular cement sheath existing between the outside surface of a pipe string and the inside formation surface or wall of a wellbore. In other cases, cements placed in wellbores are subjected to mechanical stress induced by vibrations and impacts resulting from operations.
Therefore, a need exists to be able to test the mechanical properties of cement such as the cement that is used in wellbore environments. This testing method needs to be able to accommodate the conditions that are found in the wellbore environment. The following testing method fail to provide a method of testing under these conditions.
Several testing methods have been developed to test various aspects of cement or concrete. For example, ASTM International has established the Standard Test Method for Flexural Strength of Concrete (Using Simple Beam With Center-Point Loading), Designation No. C 293-02. This test method purports to cover the determination of the flexural strength of concrete specimens by the use of a simple beam with center-point loading. The mechanism in this test employs a load-applying block and two specimen support blocks. Force is applied perpendicular to the face of the specimen until the specimen fails. The modulus of rupture is calculated as:R=3 PL/2bd2  (1)where:
R=Modulus of rupture, psi, or MPa,
P=maximum applied load indicated by the testing machine, lbf, or N,
L=span length, in., or mm,
b=average width of the specimen at the fracture, in., or mm, and
d=average depth of the specimen a the fracture, in., or mm.
This testing method only provides a modulus of rupture based on a perpendicular force being applied in surface ambient conditions. This testing method therefore fails to simulate the stresses encountered in the higher temperature and pressure conditions of the wellbore environment.
Additional standards have been developed for testing cement. For example ASTM International Standard Test Method for Flexural Strength of Hydraulic-Cement Mortars, Designation No. C 348-02 provides a centerpoint loading such that forces are applied to the specimen in a vertical direction to determine the flexural strength from the total maximum load as follows:Sf=0.0028 P  (2)where
Sf=flexural strength, Mpa, and
P=total maximum load, N.
This testing method only provides a flexural strength based on a vertical force being applied in surface ambient conditions to cause a total maximum load. This testing method therefore also fails to simulate the stresses encountered in the higher temperature and pressure conditions of the wellbore environment.
The standards also include a testing method to measure splitting tensile strength. For example ASTM International Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens, Designation No. C 496-96 provides for applying a diametrical compressive force along the length of a cylindrical concrete specimen until failure of the specimen. The loading induces tensile stresses on the plane containing the applied load and relatively high compressive stresses in the area around the applied load. Tensile failure occurs rather than compressive failure because the areas of load application are in a state of triaxial compression. The splitting tensile strength of the specimen is calculated by the formula:T=2P/(Πld)  (3)where:
T=Tensile splitting strength, psi (kPa),
P=maximum applied load indicated by the testing machine, lbf (kN),
Π=3.1416
l=length, in. (m), and
d=diameter, in. (m).
Similarly to the previously discussed testing methods, this testing method only provides a tensile splitting strength based on a diametrical compressive force applied in surface ambient conditions. This testing method therefore fails to simulate the stresses encountered in the higher temperature and pressure conditions of the wellbore environment.
Additionally, each of these standards specifically instructs the creation of the specimens at a temperature and pressure that is similar to ambient surface conditions. None of these testing methods provides for the creation of samples under the temperature and pressure conditions found in a wellbore environment.
Therefore a need exists for the formation and testing of cement under a simulation of the conditions found in a wellbore environment. Testing methods under these conditions will provide data that is more precise in providing for a method to determine the mechanical characteristics of the specimen.